The Olympian affair / Jim Butcher.

For centuries the Cinder Spires have safeguarded humanity, rising far above the deadly surface world. Within their halls, aristocratic houses rule, developing scientific marvels and building fleets of airships for defence and trade.Now, the Spires hover on the brink of open war.Everyone knows it's coming. The guns of the great airship fleets that control the skies between the last bastions of humanity will soon speak in anger, and Spire Albion stands alone against the overwhelming might of Spire Aurora's Armada and its new secret weapon - one capable of destroying the populations of entire Spires.